Description of the Related Art Many photographic cameras use built-in light metering devices, such as photodiodes and associated control circuits, for metering the illumination both before and during exposure of the film. Single Lens Reflex (SLR) cameras and other cameras having interchangeable lenses typically provide metering using the same light path as that used to expose the film.

Many cameras, especially single lens reflex cameras, use a light metering technique called off-the-film (OTF) metering. In OTF metering, a sensor in the camera detects light that is reflected either by the back of the shutter curtain (before the shutter opens) or from the film (after the shutter opens). The sensor data can be used in several ways. One use for the data includes calculating the proper camera shutter speed. Another use is to modify the intensity and duration of an electronic flash.

OTF metering techniques rely on the reflectance and scattering properties of different films (some cameras try to determine whether the film is slide film or print film using the film latitude information data (i. e., the DX value) encoded on the film cartridge).

OTF metering accuracy is limited by variations that exist between different batches of the same film. As is known in the photographic art, the process of film manufacturing involves making an mulsion and then sensitizing the mulsion using a batch process. In the case of color film, the mulsion is sensitized to obtain the desired spectral sensitivity for each color absorption layer. After an emulsion batch is sensitized, a small coating sample is made for film characterization. Each batch is analyzed and then appropriate sensitivity adjustment coatings are specified to keep the speed of the respective color layers in balance. For example, if the green layer is a bit slow, the red and blue layers are filtered to reduce sensitivity so that the film maintains proper overall color balance. The various thicknesses of the resulting coatings interact with the impinging light (by wavelength) and affect the surface reflectance of the film, batch to batch. This variation occurs in addition to the already large film surface reflectance variability across film types.

The problems of OTF metering are even more pronounced in electronic cameras that use an electronic image sensor (i. e., electronic film) in place of photographic film. Conventional camera flash units based on OTF metering systems are designed to operate with photographic film as the reflecting surface. Electronic image sensors that are used to replace photographic film are assigned ISO film speed ratings that may not be calibrated to produce optimally exposed images when used with the conventional camera flash unit. When a material such as an electronic image sensor, with reflectance and scattering properties different from conventional film, is used as the imaging device, the OTF metering system may produce a flash duration that results in an under-exposed or over-exposed image. Also, the

light sensitivity of the electronic film typically does not match that of photographic film over all incident light intensities, thus producing improper exposures in some cases. Although this problem can occur in non-flash pictures, it is particularly troublesome in flash pictures.

Another problem with electronic image sensors in conventional cameras is that random specular reflection from smooth metallic surfaces associated with the image sensor can increase the photographic flare in the image, which produces lower contrast in the resulting photographic image.

Summarv of the Invention The present invention solves these and other problems by providing a modified electronic imager system that makes the imager more compatible with OTF metering systems designed for photographic film. In one embodiment, the modified system includes a combination of reflectance-modifying treatments that make the electronic image system more compatible with the OTF metering system. Reflectance-modifying treatments include anti-reflection coatings (or materials), diffuse reflective coatings (or materials), and light absorbing coatings (or materials). The treatments are applied to the image sensor package and other surrounding components that have undesirable reflection properties. The geometrical arrangement and relative combinations of these reflectance-modifying treatments are tailored to the particular geometry of the OTF metering system as well as the sensor and electronic component placement in the electronic imager system.

The reflectance-modifying treatments can be used to reduce (or enhance) the reflection coefficient of the packaged image sensor and surrounding area. The reflectance-modifying treatments can be use to alter the polarization of the light reflected by the packaged image sensor and surrounding area. The reflectance-modifying treatments can be used to alter the phase of the light reflected by the packaged image sensor and surrounding area.

In one embodiment the reflectance-modifying treatments have a physical structure such as raised ridges, bumps, cones, grooves, etc, that modify the reflective properties of the surface. In one embodiment the reflectance- modifying treatments have a rough surface.

In one embodiment, some of the modifications are configured as an overlay or cover that is added to an existing electronic imager system.

One version includes an electronic film imager configured to be used in a conventional film camera having an off-the-film metering system. The electronic film imager includes an image-sensing array, an optical filter in front of the image sensing array, an optical absorbing material or coating around the image sensing array, and an optical diffuser. The optical filter, absorber, and diffuser modify the reflectivity of the electronic film imager so that an off- the-film metering system configured to operate with photographic film will produce properly exposed images when used with the electronic film imager.

One version includes an electronic film back to be used in place of the camera back of a conventional film camera having an off the film metering system. The electronic camera back includes an image-sensing array, an optical filter in front of the image sensing array, an optical absorbing material or coating around the image sensing array, and an optical diffuser. The optical filter, absorber, and diffuser modify the reflectivity of the electronic film

imager so that an off-the-film metering system configured to operate with photographic film will produce properly exposed images when used with the electronic film imager.

One embodiment includes a method for improving the exposure time for flash photography when an image- sensing array is used in a camera having off the film metering. The method includes placing an optical filter in front of the image-sensing array. In one embodiment, the optical filter is a transparent window with an optical coating designed to reduce reflections from the window. The method also includes placing an optical absorber near the image- sensing array to absorb some of the light that strikes the structures supporting the array. Optical diffusers can also be used to reduce specular reflections from the supporting andlor packaging structures.

One version includes an electronic film imager for use with an off-the-film metering system. The reflection properties of the electronic film imager are controlled by using an optical filter, an optical absorber, and/or an optical diffuser. The imager can be used with an electronic film cartridge configured to allow a conventional film camera, such as a single lens reflex camera, to be used as an electronic camera.

One embodiment is an electronic imaging module that can be inserted in to a photographic camera for converting the camera into an electronic imaging camera. The electronic imaging module includes an image sensor having a protective window coated with an anti-reflective coating. The electronic imaging module can also be fitted with optical absorbers (either as coatings or by an overlay) to reduce reflections from the module. The anti-reflective coatings and absorbers make the image sensor compatible with off-the-film metering systems designed for photographic film.

One embodiment includes an integrated circuit package for packaging an imaging integrated circuit die. The package includes a protective window over the die. The window has an anti-reflective coating on at least one surface.

The package can include a relatively non-reflective optical coating to reduce reflections from the package itself. The non-reflective coatings can be coated on the package or provided on a cover that fits over the package (while leaving the window exposed).

Brief Description of the Orawinos These and other features of the invention will now be described with reference to the drawings in which: Figure 1 is a schematic diagram of a camera with an OTF metering system.

Figure 2A shows a typical SLR camera with an OTF metering system; with the mirror down and the shutter closed.

Figure 2B shows a typical SLR camera with an OTF metering system; with the mirror up and the shutter open.

Figure 3A shows a typical SLR camera with an electronic film cartridge for converting the SLR into an electronic camera.

Figure 3B shows a front view of the electronic film cartridge shown in Figure 3A.

Figure 3C illustrates an electronic film back that replaces the film back on a conventional film camera.

Figure 4 is a graph showing camera flash optical intensity versus time for two different target objects, where the imaging device is photographic film and the flash is controlled by an OTF system.

Figure 5 is a graph showing camera flash optical intensity versus time for two different target objects, where the imaging device is an electronic image sensor and the flash is controlled by an OTF system.

Figure 6 shows an electronic film cartridge with absorber to reduce specular reflections that would adversely affect the operation of an OTF metering system.

Figure 7 shows an integrated circuit package configured with an absorptive coating designed to reduce reflections from the package.

Figure 8 is a graph showing camera flash intensity versus time for two different targets, where the imaging device is an electronic image sensor that has been modified by using an optical filter and an absorbing mask to reduce reflections.

Figure 9A shows a reflectance-modifying surface having ridges.

Figure 9B shows a reflectance-modifying surface having grooves.

Figure 9C shows a reflectance-modifying surface having a rough (randomized) surface.

In the drawings, like reference numbers are used to indicate like or functionally similar elements. The first digit of each three-digit reference number generally indicates the figure number in which the referenced item first appears.

Detailed Descrintion In a typical OTF metering system, as shown in Figure 1, light from an object 111 is focused by a camera lens 102 onto an imaging surface 103. The imaging surface 103 can be the back of a shutter curtain, a photographic film, an electronic image sensor, etc. Reflected light, R, from the imaging surface 103 is sensed by the sensor 110 and provided to an OTF metering circuit. The metering circuit uses a combination of a peak optical intensity and a time- integrated intensity to determine the appropriate flash duration (or shutter speed) to produce sufficient exposure (e. g., on an imaging material such as photographic film, etc.).

The OTF light measuring method works both with available light and with a flash. OTF is particularly useful for flash pictures because it allows metering to be provided during the exposure process. OTF metering uses the sensor 110 to measure light scattered from the film when the shutter is open (for faster shutter speeds, the light is scattered by both the film and a pattern provided on the back of the first shutter curtain).

Figures 2A and 2B show a single lens reflex (SLR) camera 200 with an OTF metering system. The camera 200 includes a mirror 205, a shutter 204, and film (either photographic film or electronic film) 203 behind the shutter.

The camera 200 also includes an OTF sensor 210 attached to an OTF metering system (not shown).

In Figure 2A, the mirror 205 is down and the shutter 204 is closed. In Figure 2B, the mirror is up and the shutter 204 is open. The camera 200 also includes a lens 202, and a penta-prism 214. Light enters the lens 202. The light is reflected by the mirror 205 and into the penta-prism 214. The penta-prism 214 directs the light into the photographer's eye 216. When the photographer takes a picture, the mirror 205 flips up allowing light from the lens

202 to fall onto the back of the shutter 204. Once the mirror is up, the shutter 204 opens allowing light from the lens 202 to fat ! onto the film 203. Light reflected andlor scattered by the film 203 falls on the sensor 210 and is used by the OTF system to control the exposure time. In non-flash photography, the OTF system can control exposure by closing the shutter when a proper exposure has been achieved. In flash photography, the OTF system can control exposure by quenching the flash once a proper exposure has been achieved.

If the film 203 is highly reflective, then the sensor 210 will receive more light and exposure times will be relatively shorter. Conversely, if the film 203 is not very reflective, then the sensor 210 will receive less light, and exposure times will be relatively longer.

In some OTF systems, a special pattern on the back of the shutter 204 is used by the OTF metering system to measure the light before the shutter 204 opens. Once the shutter 204 is open, the OTF system compares the intensity of the light reflected from the film to the light reflected by the shutter 204 and adjusts the exposure time based on the difference between the light reflected by the shutter 204 and the light reflected by the film 203.

Figure 3A shows the camera 200 with an electronic film cartridge 302. The cartridge 302 is configured to convert the camera 200 into an electronic film camera. The cartridge has a"flag"portion that fits across the back of the camera 200 behind the shutter 204 (not shown) and a cylindrical portion that fits in a film holder of the camera 200. The cartridge 302 includes an electronic image sensor (in the flag portion) and processing and storage circuits needed to capture and store images from the electronic image sensor. Figure 3B shows a front view of the cartridge showing a window 310 over the electronic image sensor and a package 311 that holds the electronic image sensor. A region 315 on the flag portion corresponds to the portion of the cartridge 302 that is exposed to light when the shutter 204 is open.

Figure 3C shows an alternative method for converting a conventional film camera into an electronic film camera. In Figure 3C the camera back 303 is replaced by an electronic film camera back 308. Like the electronic film cartridge 302, the electronic film camera back 308 includes the window 310 over the electronic image sensor and a package 311 that holds the electronic image sensor. A region 315 on the camera back 308 corresponds to the portion of the camera back 308 that is exposed to light when the shutter 204 is open. In the discussion that follows, the electronic film cartridge 302 is used as an example, with the understanding that the electronic camera back 308 can be used in the alternative.

The electronic film cartridge 302 converts the camera 200 into an electronic camera. However, if the electronic film cartridge 302 is not properly designed, the OTF metering system in the camera 200 will not work well with the electronic film cartridge 302 to produce properly exposed images. This is especially true when the camera 200 is used to take flash pictures and the OTF system is used to quench the flash when a proper exposure has been achieved.

Figure 4 is a time-resolved plot of the optical intensity of a flash controlled by an OTF metering system when photographic film is used as the imaging surface 103. Figure 2 shows a curve 402 corresponding to the optical intensity produced by aluminum foil (a highly reflective object), and a curve 404 corresponding to the optical intensity

produced by an 18% matte gray background (a relatively non-reflective object). The curves 402 and 404 show that the peak optical intensity and the overall flash duration are parameters that can be varied based on reflected light input to the OTF metering system. The curve 402 shows a higher peak flash intensity and duration than the curve 404.

Each curve shows a peak intensity followed by a relatively slow decay and then a"knee"that leads into a relatively rapid decay in the intensity. In each curve, the height of the peak intensity and the temporal location of the"knee"in the optical intensity curve can be controlled by the OTF metering circuit.

Figure 5 is similar to Figure 4, except that Figure 5 shows intensity curves when an electronic image sensor is used in place of the photographic film used in Figure 4. Figure 5 shows a curve 502 corresponding to an aluminum target, and a curve 504 corresponding to an 18% matte gray target.

Although the peak intensity and flash duration of the curves 502 and 504 are similar to the curves 402 and 404 shown in Figure 4, the flash characteristics shown in Figure 5 lead to under-exposed images due to the difference in optical sensitivity between the photographic film and the electronic image sensor. The images are underexposed, in part, because portions of the electronic image module have a relatively higher reflectance value than the photographic film and thus tend to direct excess light into the OTF sensor 110-causing the OTF meter to quench the flash prematurely.

The reflective properties of an electronic imager can be improved by adding reflectance-modifying treatments in the form of coatings, materials, or surfaces to the electronic imager. In one embodiment, the reflectance-modifying treatments include a combination of anti-reflection coatings, diffuse reflective coatings, and light absorbing coatings to the image sensor package and any other surrounding components with high specular reflection. The geometrical arrangement and relative combinations of these reflectance modifying materials can be tailored to the particular geometry of a selected OTF metering system as well as the sensor and electronic component placement in the electronic image capture system.

Reflectance-modifying treatments (e. g., coatings, materials, surfaces, etc.) also include treatments that change the polarization (e. g. linear, circular, or elliptical polarization) of light reflected by the surface. Reflectance- modifying treatments also include treatments that change the phase of the reflected light.

The higher reflectance of the electronic image module can be controlled by using anti-reflective coatings on the window 310, and by using absorptive coatings and diffusing coatings on the other portions of the electronic image module.

For example, Figure 6 shows the electronic film cartridge 302 (from Figure 3B) with the addition of an absorber coating (shown as a hatch) disposed in the region 315 around the window 310. The absorber coating reduces the reflections produced by the electronic film cartridge 302 that might confuse an OTF metering system. The window can be covered by a bandpass filter that absorbs light outside the visible range. In addition, or in the alternative, the faces of the window and can be coated with an anti-reflective coating.

Figure 7 shows an exploded view of an integrated circuit package 700 for an image sensor die 710. The package 700 includes a reflectance-modifying treatments to make the packaged sensor more compatible with OTF

metering systems. The package 700 includes a window 310, an upper frame 701 having an upper surface 708, a lower frame 703 having an upper surface 702, and a back cover 704. The die 710 is attached to the lower frame 703 and electrical contacts on the die 710 are provided to electrical contacts on the lower frame 703. The back cover 704 protects the back side of the die 710. The upper frame 701 is attached to the lower frame 703, and the upper frame 701 supports the window 310. The upper surface 702 of the upper frame 701 can be used as an optical reference frame to align an imaging surface of the die 710 with an image plane of a camera or other optical system.

In one embodiment, reflectance-modifying treatments include coatings such as an anti-reflection coating on one or both surfaces of the window 310 to reduce reflections from the window 310. The anti-reflection coating can be implemented as a bandpass filter. In one embodiment, a reflectance-modifying treatment such as absorber coating on the upper surfaces 708 and 704 reduces reflections from the upper frame 701 and lower frame 703. The absorber can be applied to the surfaces 704,708, integrated into the frames 701,703, or provided on a mask (or cover) that fits over the frames 701,703. A diffuser (either a diffuser material or structure) can also be provided to reduce specular reflections from the frame 701 (as described in connection with Figures 9A-9C below).

Suitable anti-reflection coating materials include magnesium fluoride. Suitable absorber coatings include dark (e. g., black) paints, dark materials, baffles, carbon particles dispersed in a liquid, etc. Suitable diffuser materials include barium titanate, titanium dioxide particles, rough surfaces, and the like.

Figure 8 shows a flash intensity profile when the coatings described in connection with Figures 6 and 7 are used with an electronic image sensor. Figure 8 shows a curve 810 corresponding to a sensor without coatings. Figure 8 also shows a curve 820 corresponding to a sensor with coatings to modify the amount of light that is reflected into the OTF metering sensor. The coatings in the curve 820 include the combination of an optical bandpass filter placed in front of the image sensor and a black, light-absorbing, mask placed around the perimeter of the image sensing device.

As shown in Figure 8, a higher peak intensity and longer flash duration are obtained which result in correctly exposed images for both targets.

In one embodiment, the particular combination of optical filtering and masking is tailored to a desired camera body and OTF metering system. For this reason, one embodiment includes a filterlmask overlay developed for each application (i. e., camera) that would optimize exposure characteristics for that camera. These masks can be permanently attached to the image sensor package or applied as a cover (overlay) that can be removably attached to the image sensor package.

The reflectance-modifying surfaces, such as the surfaces 704,708 shown in Figure 7 can be smooth or structured. Figure 9A shows a structured reflectance-modifying surface having ridges. In an alternative embodiment, the ridges are structured as cones.

Figure 9B shows a structured reflectance-modifying surface having grooves running along the length of the surface. In an alternative embodiment, the grooves run in two directions (e. g., horizontal grooves and vertical grooves) to produce rectangular (or cubical) protrusions on the reflectance-modifying surface.

Figure 9C shows a structured reflectance-modifying surface configured as a rough (randomized) surface to produce a diffuse specular reflection property.

The structured surfaces shown in Figures 9A-9C can be coated as described above to produce a dark structured surface having a desired reflection coefficient. In one embodiment, the desired reflection coefficient is relatively small and produces a diffuse reflection.

Although this invention has been described in terms of a certain embodiment, other embodiments apparent to those of ordinary skill in the art also are within the scope of this invention. Various changes and modifications may be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is defined by the claims that follow.